The invention concerns an illumination system for wavelengthsxe2x89xa6193 nm, i.e., VUV and EUV-lithography with a plurality of light sources, for example, as well as a mirror or lens device for producing secondary light sources, comprising several mirrors or lenses, divided into raster elements.
In order to allow even further reduction in the structural width of electronic components, especially to the submicron range, it is necessary to reduce the wavelength of the light used in microlithography.
For wavelengths smaller than 193 nm, lithography with weak x-rays or so-called EUV-lithography is discussed.
A suitable illumination system for EUV-lithography should homogeneously or uniformly illuminate, with as few reflections as possible, a pregiven field for EUV-lithography, especially the annular field of an objective lens, under lithography requirements. Furthermore the pupil of the objective lens should be illuminated up to a particular degree of filling "sgr", independently of the field, and the exit pupil of the illumination system should be situated in the entrance pupil of the objective lens.
Regarding the basic layout of EUV-illumination systems, we refer to the applicant""s pending applications EP 99 106348.8, submitted on Mar. 2,1999, entitled xe2x80x9cIllumination system, especially for EUV-lithographyxe2x80x9d, U.S. Ser. No. 09/305,017, submitted on May 4, 1999 entitled xe2x80x9cIllumination system particularly for EUV-lithographyxe2x80x9d, and PCT/EP 99/02999, submitted on May 4, 1999, entitled xe2x80x9cIllumination system, especially for EUV-lithographyxe2x80x9d, whose disclosure contents are incorporated in their entirety in the present application.
The following are discussed herein as light sources for EUV-illumination systems:
laser plasma sources
pinch plasma sources
synchrotron radiation sources
In the case of laser plasma sources, an intensive laser beam is focused onto a target (solid, gas jet, droplet). The target is heated so strongly by the excitation that a plasma is formed. This emits EUV-radiation.
Typical laser plasma sources have a spherical beam, i.e., a radiation angle of 4xcfx80, as well as a diameter of 50 xcexcm to 200 xcexcm.
In pinch plasma sources, the plasma is produced by means of electrical excitation.
Pinch plasma sources can be described as volume radiators (D=1.00 mm), which emit in 4xcfx80, whereby the beam characteristic is dictated by the source geometry.
In the case of synchrotron radiation sources, one can distinguish three types of sources at present:
bending magnets
wigglers
undulators
In bending magnet sources, the electrons are deflected by a bending magnet and emit photon radiation.
Wiggler sources comprise a so-called wiggler for deflection of the electron or an electron beam, and this wiggler comprises a multiple number of alternating polarized pairs of magnets arranged in rows. If an electron passes through a wiggler, it is subjected to a periodic, vertical magnetic field and the electron oscillates in the horizontal plane. Wigglers are also characterized by the fact that no coherency effects occur. The synchrotron radiation produced by a wiggler is similar to a bending magnet and radiates in a horizontal solid angle. In contrast to the bending magnet, it has a flux that is intensified by the number of poles of the wiggler.
There is no clear dividing line between wiggler sources and undulator sources.
In case of undulator sources, the electrons in the undulator are subjected to a magnetic field of shorter period and smaller magnetic field of the deflection poles than in the case of a wiggler, so that interference effects occur in the synchrotron radiation. The synchrotron radiation has a discontinuous spectrum based on the interference effects and emits both horizontally as well as vertically in a small solid-angle element; i.e., the radiation is highly directional.
It is critical for an EUV-illumination system to provide a sufficiently high Lagrange optical invariant or etendu. The Lagrange optical invariant of a system is defined as the product of the illuminated surface times the square of the aperture.
If the numerical aperture in the plane of the wafer is in the range NAwafer=0.1-0.25, then in the case of 4:1 systems, a numerical aperture in the reticle plane of NAreticle=0.025-0.0625 is needed. If the illumination system is supposed to illuminate this aperture homogeneously and independent from the field up to a filling degree of "sgr"=0.6, for example, the EUV-source must have the following 2-dim Lagrange optical invariant or etendu: (LC).
xe2x80x83LCill.="sgr"2LCObj=0.149 mm2-0.928 m2
The Lagrange optical invariant LC, is generally defined as follows for the lithography system described herein:
LCill.="sgr"2xxc2x7yxc2x7NA2="sgr"2Axc2x7NA2,
wherein A is the illuminated area. The area comprises 110 mmxc3x976 mm, for example, in the reticle plane.
The Etendu of a laser plasma source is defined as the product of the illuminated surface of an imaginary unit sphere around the source and the square of the Numerical Aperture at which each field point of the imaginary unit source sees the spherical source.
LC=Axc2x7NA2
ALPQ=2xcfx80[cos(xcex81)xe2x88x92cos(xcex82)]xc3x97(Rsphere)2, with Rsphere=1 mm
NA≈rLPQ/Rsphere=0.100
where xcex81 is the minimum beam angle with respect to the optical axis and xcex82 is the maximum beam angle with respect to the optical axis
LCLPQ=2xcfx80[cos(xcex81)xe2x88x92cos(xcex82)]xc2x7r2LPQ
With the typical source parameters:
1. rLPQ=0.1 mm, xcex81=0xc2x0, xcex82=90xc2x0 yields: LCLPQ=0.063 mm2. This corresponds to 27% of the required value of the Lagrange optical invariant LCill of, for example, 0.236 mm2.
2. rLPQ=0.025 mm, xcex81=0xc2x0, xcex82=90xc2x0 yields: LCLPQ=0.0039 mm2. This corresponds to 1.7% of the required value of the Lagrange optical invariant of, for example, LCill=0.236 mm2.
The Lagrange optical invariant LCPinch of a pinch plasma source with a diameter of 1 mm, xcexa9=0.3 sr, for example, is:
LCPinch=Axc2x7NA2=(xcfx80xc2x71 mm2/4)xc2x70.30532=0.073 mm2.
Thus, the pinch plasma source provides 31% of the required value of the Lagrange optical invariant of, for example, LCill=0.236 mm2.
The Lagrange optical invariant or Etendu for the undulator source can be estimated by a simplified model assuming a homogeneous two-dimensional radiator with diameter
Ø=1.0 mm and aperture NAUnd=0.001 with
LCUnd=Axc2x7NA2 
AUnd=xcfx80xc2x7(Ø/2)2 
=0.785 mm2 
NAUnd=0.001
as
LCUnd=Axc2x7NA2=0.00000079 mm2=7.9e-07 mm2.
As can be seen from this rough calculation the Etendu or Lagrange optical invariant of the undulator source is much too small in comparison to the required value of the Lagrange optical invariant.
To increase the Lagrange optical invariant, an illumination system comprising a synchrotron radiation source known from U.S. Pat. No. 5,512,759, comprises a condenser system with a plurality of collecting mirrors, which collect the radiation emitted by the synchrotron radiation source and form it to an annular light beam that corresponds to the annular field being illuminated. By this, the annular field is illuminated very uniformly. The synchrotron radiation source has a beam divergence greater than 100 mrad in the plane of radiation.
U.S. Pat. No. 5,439,781 shows an illumination system with a synchrotron radiation source, in which the Lagrange optical invariant, is adjusted by means of a scattering plate in the entrance pupil of the objective lens, wherein the scattering plate can comprise a plurality of pyramidal structures. Also, in U.S. Pat. No. 5,439,781, the synchrotron radiation source has a beam divergence greater than 100 mrad. The synchrotron radiation according to U.S. Pat. No. 5,439,781 is also focused, for example, by means of a collector mirror.
The disclosure contents of both of the above-mentioned documents
U.S. Pat. No. 5,512,759
U.S. Pat. No. 5,439,781
are incorporated into the disclosure contents of the present application by reference.
The object of the invention is to supply an illumination system of easy construction having the required Etendu in the objectxe2x80x94or reticlexe2x80x94plane.
The object of the invention is solved in that several light sources are coupled in order to illuminate the exit pupil of the illumination system up to a predetermined degree of filling.
The coupling of several light sources also results in an increase of intensity. A coupling of several light sources is possible as long as the total Lagrange optical invariant of all coupled sources is less than the Lagrange optical invariant of the illumination system LCill.
There are three basic possibilities for coupling:
1. Addition method: Identical or similar illumination systems are distributed about an axis of the system. The exit pupil of the illumination system is illuminated by the circular pupils of the system parts, which must not overlap. The partial pupils are located on the side face of a pyramid-shaped input mirror, which superimposes the light bundles on the object or reticle.
2. Mixing method: In this case, each system part illuminates the entire exit pupil of the illumination system, but with regions free of light between the secondary light sources. The individual grids of the secondary light sources are staggered by superimposing, so as to uniformly fill the pupil. The coupling mirror consists of a raster element plate, whose raster elements or facets have the shape of pyramids. Each side face of an individual raster element pyramid is illuminated by a secondary light source.
3. Segment method: Similar to the addition method. Unlike the addition method, a segment of any desired shape is illuminated by appropriate beam deflection, instead of a circular segment of the exit pupil.
Preferred embodiments of the invention making use of at least one of the above-mentioned methods are the subject of the subsidiary claims.